Xanathar's Guide to Everything (p. 157) features the spell Illusory Dragon. Maybe it's me, but doesn't the spell description scream Sorcerer (especially Draconic Bloodline)? Imagine my surprise when I saw that the spell is only available to Wizards.
Would it be balanced to allow this spell for Sorcerers?
Maybe I am missing some extreme synergy with a Sorcerer class feature that would make it overpowered...
It's a really high level spell, and I doubt it's on very many Wizards' short-list of spells they would choose on level-up. It screams Illusionist to me, but I could certainly see a Draconic Bloodline sorcerer having it, even if just for flavor. It doesn't seem unbalanced at all with other Sorcerer Level 8 spells, and you will have to take it instead of a different spell that you'd take/swap between levels 15 and 20.
To add to this, even the DMG (p. 287) says changing spell lists isn't generally a problem, except for the Warlock spell list due to short rest recovery:
Modifying a class’s spell list usually has little effect on a character’s power but can change the flavor of a class significantly.
Which I suspect is exactly what you want. No problem at all.
